1. Field of the Invention
This invention relates to a copolymer obtained by copolymerizing specific monomers and a resin for electrophotographic functional component parts, which makes use of the same. It also relates to a developing roller and an electrophotographic apparatus.
2. Related Background Art
Electrophotographic apparatuses utilizing a dry-process electrophotographic system, such as copying machines, laser beam printers, facsimile machines, printers, and the like, are known in the art. In an image forming apparatus, such as an electrophotographic apparatus, the surface of an electrostatically charged image bearing member (hereinafter also “photosensitive member”) is electrostatically and uniformly charged, then an electrostatically charged image is formed on the surface thereof by an electrostatically charged image forming unit including an exposure means, and this electrostatically charged image (latent image) is developed as a toner image by means of a developing assembly. A charging member, which charges the photosensitive member electrostatically, a developing member, which feeds a developer to the electrostatically charged image, a toner feed member, which feeds the developer to the developing member, and so forth, which are provided in such an electrophotographic apparatus, are required to have an appropriate flexibility and are made from a urethane resin having a rubber elasticity. In order to achieve good nips in these electrophotographic functional component parts, it is preferable for the urethane resin to have a low hardness. However, using a low hardness urethane resin makes these component parts tacky, what is called stickiness, on their surfaces. Toner does not separate well from such component parts having a high surface tackiness. Thus, the toner may adhere to the surfaces of component parts during use and negatively affect their characteristics.
To reduce the surface tackiness, proposed are, e.g., a method in which dimethylsiloxane is mixed (Japanese Patent Laid-open Application No. H08-296631), a method in which the proportion of a polyol component with respect to an isocyanate component is controlled (Japanese Patent Laid-open Application No. H09-114190) and a method in which a fluorine resin is mixed (Japanese Patent Laid-open Application No. 2001-042663).
Meanwhile, to obtain a urethane resin for electrophotographic functional component parts by a urethanation reaction of a polyol component with an isocyanate component, a catalyst, such as an amino compound, is required in many cases. However, in component parts formed using such a urethane resin, which was obtained using the amino compound as a catalyst, the remaining amino compound may exude during use to stain other component parts and cause difficulties.
In order to keep such other component parts from being stained because of the amino compound catalyst remaining in the urethane resin for electrophotographic functional component parts, a method is proposed in which a catalyst having a hydroxyl group in the molecule is used (Japanese Patent Laid-open Application No. 2005-113057).
However, no resin for electrophotographic functional component parts has been proposed that can both reduce surface tackiness and prevent staining caused by the exuded amino catalyst compound, which is used for the polymerization reaction. Thus, there is a demand for a resin that can solve these problems simultaneously.
A developing roller as the developing member is required to uniformly and sufficiently triboelectrically charge the toner and further transport the toner while holding it thereon. However, with electrophotographic apparatuses becoming high-speed and high-image quality devices in recent years, the deterioration of toners has accelerated. Specifically, the toner tends to stick to the developing roller surface and may be insufficiently fed to the electrostatically charged images to cause a decrease in image density or fogging. Accordingly, a developing roller is needed that has an improved triboelectric charging performance with respect to the toner, can increase the toner charge, and can perform sufficient triboelectric charging even when a toner has deteriorated.
The developing roller has an electrically conductive shaft and an elastic layer provided on its peripheral surface. It also optionally includes a cover layer on the surface. Such a developing roller is commonly used. The surface of the developing roller must be formed using a negatively chargeable material where the toner is to be positively charged and a positively chargeable material where the toner is to be negatively charged. Known in the art are a method in which a charge control agent is added to a resin that forms a surface layer and a method in which a surface layer is formed of a charge control resin. Stated specifically, a method is reported in which a coating film is formed on the developing roller or sleeve by using a charge control resin soluble in an organic solvent (Japanese Patent Laid-open Applications Nos. 2000-242033, 2002-244426 and 2003-005507). In all of these documents, in the coating film formed on the developing roller or the like, a copolymer chiefly made from an amino group-containing acrylate or a methacrylate monomer and methyl methacrylate is used as the charge control resin.
In a developing roller intended to improve image characteristics by increasing the charge of the toner, the present inventors have reported that a positive charge control resin is used to which an acrylate or a methacrylate copolymer having an alkyl group having 4 or more carbon atoms with an amino group-containing compound has been added (Japanese Patent Laid-open Applications Nos. 2005-031656 and 2005-031657). In this developing roller using such a positive charge control resin, the alkyl group having 4 or more carbon atoms that is contained in the acrylate or methacrylate unit promotes the surface orientation of the amino group contained in the amino group-containing monomer unit to improvement negative chargeability of the toner.
In this developing roller, image characteristics can be recognized to have been improved because of an increase in the negative charge quantity of the toner by adding the above positive charge control resin to a surface layer. However, difficulties in image characteristics, which are presumed to be caused by the charge-up of the toner, such as blotches, may come about depending on image evaluation conditions.